The present invention relates generally to printing or marking apparatus, and is particularly concerned with a printing apparatus in which a reciprocating member carries a printing element that is inked by means of a rockably mounted inking roll.
Various types of printing or coding devices have been developed for printing variable information on products, product containers, or continuous webs of product wrapping material. Generally, these devices are designed for installation on existing product or web conveyor systems, container filling systems, and similar systems, and are used to print date codes, lot or batch numbers, or other types of information. The simplest type of product coding device is a rotary printer, in which a rotary die roll or type holder carries a printing element that is brought into rolling contact with the article or web surface to be printed. These devices require that the product or web be kept in motion, either continuously or during the time that the printing element is in contact with the surface to be printed. Inking of the printing element is usually carried out by mounting a rotatable inking roll in a fixed position next to the rotatable die roll or type holder.
In situations where an intermittently moving conveyor is used to transport the articles or web to be printed, as in the case where a container filling or sealing operation requires periodic stopping of the conveyor, it may be more convenient to use a reciprocating or stamping printer rather than a rotary printer. The temporary stopping of the conveyor provides an interval during which printing can be carried out by a reciprocating printing member, and inking of the printing element can occur before, during or after the printing operation by means of an inking roll or other type of inking device.
The inking of the printing element in a reciprocating printer is somewhat more difficult than in the case of a rotary printer, particularly when an inking roll is used as the inking device. In a rotary printer, the inking roll can be mounted with its axis in a fixed position with respect to the axis of the rotating printing member, and this will suffice to apply ink to the printing element by rolling contact once during each printing cycle. In a reciprocating printer, however, it is generally not possible to mount the inking roll in a fixed position relative to the path of the printing member, since this will cause ink to be applied to the printing elements twice during each printing cycle, possibly resulting in too much ink being applied. In addition, the path of movement of the reciprocating printing member may be such that it would be physically obstructed by the inking roll at some point during the printing cycle if the inking roll were to be held in a fixed position. For these reasons, it is common in reciprocating printers to mount the inking roll in a rocking or oscillating structure, so that the inking roll can be brought into rolling contact with the printing elements only once during each printing cycle and then retracted from the path of the printing member.
An additional problem that arises in reciprocating or stamping printers has to do with the fact that the printing element is usually in the form of a plane, such as a flat printing plate or several rows of type carried by a flat type holder. Clearly, if a flat printing element is to be inked by contact with a cylindrical inking roll, the printing element must move tangentially along the periphery of the inking roll (or vice-versa) during the period in which inking is taking place. During the printing interval, however, the printing member must move in a direction normal to the plane of the printing element in order to bring the printing element into contact with the surface to be printed. In order to satisfy both of these requirements, as well as the additional requirement that the printing element have ink applied to it only once during each cycle of movement of the printing member, a complex motion of either the inking roll or printing member is usually needed.
The problem of maintaining tangency between a flat printing element and an inking roll in a reciprocating printing or coding device has been addressed in the prior art. In commonly assigned U.S. Pat. No. 4,444,108, issued to Peter Jenness, III on Apr. 24, 1984 and incorporated herein by reference, the printing member moves in an arcuate path between a first position in proximity to a surface to be printed and a second position remote from the surface, under the control of a rotary crank mechanism. An inking roll assembly is mounted for rocking movement along a path which intersects the arcuate path of the printing member. The inking roll assembly is cyclically rocked in timed relation to the movement of the printing member by a cam and follower arrangement connected to the rotary crank mechanism. The cam profile is preferably chosen in such a manner as to accomplish two functions, one being to ensure that the inking roll is brought into contact with the printing element on the printing member only once during each back-and-forth cycle of movement of the printing member, and the other being to ensure that the inking roll is maintained in uniform tangential contact with the flat printing element during the interval when these two components are in physical contact.
Although the printing apparatus described in U.S. Pat. No. 4,444,108 has been commercially successful, a number of limitations exist on the reliability and performance of the apparatus. For example, two different coil springs are used in the apparatus, one operating in tension to maintain the inking roll cam follower in contact with the inking cam, and the other operating in compression as part of a resilient support means which allows the printing member to move vertically downward into contact with the surface to be printed when it is held against rotation by a stop member at a certain point during the printing cycle. Both of these springs are subject to fatigue and breakage after prolonged operation of the printing apparatus, leading to undesirable down time while the springs are replaced. The manner in which the printing member is caused to move vertically downward by the rotary crank mechanism is also somewhat disadvantageous, since at higher printing speeds the abrupt striking of the printing member against the stop member causes the operation of the printing apparatus to become uneven. As a practical matter, this limits the maximum printing speed to about 100 cycles per minute in commercial embodiments of the printing apparatus. It has also been found that the linear bearings and guide shafts which are used to guide the movement of the printing member during its vertical travel into contact with the surface to be printed are subject to wear, resulting in a slight lateral motion of the printing member during printing and consequent distortion of the printed image.
A number of other problems have also been encountered with the printing apparatus described above. For example, it is usually desirable to rock the inking roll in such a way as to initiate and withdraw contact between the inking roll and printing element rather abruptly at the leading and trailing edges of the printing element, since a more gradual rate of inking roll movement at either of these edges may result in ink being applied to (and accumulating on) the sides of the printing element or to its mounting structure. Since the speed at which the inking roll rocks toward or away from the printing element will depend upon the rate of change of the cam radius at that point, it can be readily seen that an abrupt motion of the inking roll will require a steeply ascending or descending cam profile. It is difficult to cause a cam follower to maintain contact with a cam surface in such regions unless the spring force applied to the follower arm is very high, which increases overall rotational resistance and worsens the problem of spring failure referred to previously.
A further problem with the printing apparatus of U.S. Pat. No. 4,444,108 has to do with the design of the drive mechanism that is used to move the printing member along its arcuate and linear paths. In cases where the work table supporting the article or web to be printed is adjusted too high, as often occurs inadvertently when the operator is attempting to adjust the printing pressure to ensure a suitably dark and uniform print, the rotary crank mechanism will exert too much pressure against the work table during its downward stroke. At high printing speeds, this gives rise to repeated mechanical shocks which are transmitted back through the printing member and crank mechanism to the other parts of the printing apparatus, resulting in premature wear of bearings, bushings and other components. The misadjustment of the work table is not always apparent to the operator, and hence the condition may persist for an extended period of time before it is corrected.
A variation of the printing apparatus described above is disclosed in Japanese Kokai Patent No. 63-315282. In the modified apparatus, the rotatable cam driven by the rotary crank mechanism serves only to rock the inking roll assembly between two discrete positions, one allowing contact between the inking roll and the printing elements and the other preventing such contact. A separate cam surface, in the form of a fixed cam slot disposed along the path of the printing member, is provided to control the motion of the printing member as the latter is reciprocated by the rotary crank mechanism. The fixed cam slot defines a combined arcuate and linear path for the printing member that allows for inking and vertical stamping, and also provides for tangency between the printing element and inking roll when these two components are in contact with each other. In order to allow the printing member to move both arcuately and linearly under the control of the rotary crank mechanism and fixed cam slot, and to maintain tangency with the inking roll, the printing member consists of two parts connected by linear bushings or bearings. One part is pivotably supported by the frame of the printing apparatus, and the other part (which carries the printing element) is pivotably connected to the connecting link of the rotary crank mechanism. The use of a fixed cam slot produces a somewhat smoother motion of the printing member than the resilient mounting assembly described previously, but linear bearings and guide shafts are still required to produce the combined arcuate and linear motion of the printing member under the control of the cam slot. As noted earlier, this arrangement is subject to wear and can cause distorted prints. Moreover, since the function of maintaining tangency between the inking roll and the printing element is still confined to a single component (in this case the printing member rather than the inking roll), the distances and speeds at which the component must be moved to maintain tangency are greater than might be desired. This can result in excessive dynamic loads and accelerated wear at high printing speeds, particularly when physically large and heavy printing elements are used.